The HierarchyExpandButton is part of the GridViewRow and in order to change it you should edit the GridViewRow's template and set its Visibility property to Collapsed. If you want to keep the synchronization between the GridViewRow and GridViewHeaderRow you should also edit the template of GridViewHeaderRow. Find the part HierarchyIndentPresenter and set its Visibility property to Collapsed too. Attached is a sample project that demonstrates this approach.
If you need any additional information please let me know.
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